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8-1-1997
Ar-40/Ar-39 Evidence for Middle Proterozoic
(1300-1500 Ma) Slow Cooling of the Southern
Black Hills, South Dakota, Midcontinent, North
America: Implications for Early Proterozoic P-T
Evolution and Posttectonic Magmatism
Daniel K. Holm
Peter S. Dahl
Daniel R. Lux
University of Maine - Main, [email protected]
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Repository Citation
Holm, Daniel K.; Dahl, Peter S.; and Lux, Daniel R., "Ar-40/Ar-39 Evidence for Middle Proterozoic (1300-1500 Ma) Slow Cooling of
the Southern Black Hills, South Dakota, Midcontinent, North America: Implications for Early Proterozoic P-T Evolution and
Posttectonic Magmatism" (1997). Earth Science Faculty Scholarship. Paper 12.
http://digitalcommons.library.umaine.edu/ers_facpub/12
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TECTONICS,VOL. 16,NO.4, PAGES609-622,AUGUST1997
4OAr/9Arevidencefor Middle Proterozoic(1300-1500Ma)
slowcoolingof the southernBlack Hills, SouthDakota,
midcontinent,
North America: Implicationsfor Early
ProterozoicP-T evolutionand posttectonic
magmatism
DanielK. Holm andPeter S. Dahl
Department
ofGeology,
KentState
University,
Kent,Ohio
Daniel R. Lux
Department
ofGeological
Sciences,
University
ofMaine,Orono
Abstract.
4øAr?9Ar
totalgasandplateau
dates
frommuscovite 1.
andbiotitein the southernBlack Hills, SouthDakota, provide
evidence
for a periodof Middle Proterozoicslow cooling. Early
Proterozoic(1600-1650 Ma) mica dates were obtained from
metasedimentary
rockslocatedin a synforma!structurebetween
theHarneyPeak and Bear Mountain domesand also southof
BearMountain. Metamorphic rocks from the dome areasand
undeformedsamplesof the - 1710 Ma Harney Peak Granite
(HPG)yield Middle Proterozoicmica dates (-1270-1500 Ma).
Twosamplescollectedbetweenthe synformandBear Mountain
domeyield intermediatetotal gasmica datesof-1550 Ma. We
suggesttwo end-memberinterpretationsto explain the map
pattern
of coolingages:(1) subhorizontal
slowcoolingof an area
Introduction
The dominantPrecambriantectonicelementsmakingup the
southern portion of Laurentia (between the Grenville and
Cordilleran Orogens, Figure 1) were rapidly formed and/or
assembledtoward the end of the Early Proterozoic(1900-1600
Ma). Preferentially intruded into the Early Proterozoic
collisional/accretionary belts of this region are numerous
anorogenic or posttectonic plutons whose origins remain
controversial[Hoffman, 1989a; Winalley, 1993; Nyman et al.,
1994;Holm and Lux, 1996]. The purposeof this studyhasbeen
to investigatethe postcollisionalintrusiveand thermalhistoryof
Early Proterozoic metamorphic rocks in the Black Hills of
whichexhibitsvariation in mica Ax retentionintervalsor (2) mild
foldingof a Middle Proterozoic(-1500 Ma) ~300øC isotherm.
According
to the secondinterpretation,the preservationof older
datesbetweenthe domesmay reflectreactivationof a preexisting
synformal
structure(and downwarpingof relativelycold rocks)
duringa periodof approximatelyeast-westcontractionand slow
upliftduringthe Middle Proterozoie. The mica data, together
withhornblendedata from the Black Hills publishedelsewhere,
indicate
thattheambientcountry-rocktemperature
at the3-4 kbar
depthof emplacementof the HPG was between 350øC and
southwesternSouth Dakota, United States of America. On the
beneath
theBlackHills. If so, thisprocess
wouldhavealso
BowffngandKarlstrom,1990;Hodgeset al., 1994;Williamsand
caused an increase in mantle heat flux across the Moho and
Karlstrom, 1996]. Reconstructionof pressure-temperature-time
triggered
vapor-absent
meltingof biotiteto producethe HPG
pathsof Proterozoie
juvenile rocksin the southwestern
United
magma. This scenariofor posttectonicgranite generationis
supported,
in part,by thefact thatin thewholeof theBlackHills,
States has led to recognition of their long-term midcrustal
residencethroughouta significant portion of the Proterozoic.
Evidencein that regionfor protracted(300-400 m.y.) midcrustal
basisof regional geophysicalfeatures,the crystallinebasement
exposedin the Black Hills representsthe southermost exposure
of the Early Proterozoic Trans-Hudson orogen near its
terminationby the youngerCentral Plainsorogen(localityBH,
Figure 1). However,theBlack Hills representthe only exposure
of the geophysically
definedTrans-Hudsonorogenicbelt for over
1000 km along strike, and the lithologic units and timing of
tectonicandintrusiveeventsthereappearfundamentallydifferent
from muchof the Trans-Hudsonorogenin Canada[i.e., Redden
500øC,suggesting
that the averageuppercrustalgeothermal and DeWitt, 1996]. For instance,the -1710 Ma posttectonic
gradient was 25ø-40øC/km prior to intrusion.
The
Hamey Peak Granite in the Black Hills is 60-70 m.y. younger
thermochronologic
datasuggest
HPG emplacement
wasfollowed than posttectonicgranitesand pegmatitesin the Saskatchewan
bya-200 m.y. periodof stabilityandtectonicquiescence
with
(Canada)segmentof the orogen[Bickfordet al., 1990], leading
littleuplift. We proposethat crustthickenedduringtheEarly
some to suggestthat the Harney Peak Granite may actually be
relatedto CentralPlainsorogensuturing[Simset al., 1991].
Proterozoic
was upliftedand erosionally(?)
thinnedprior to
During the last decade, new and improved petrologic and
~1710Ma and that the HPG magma was emplacedinto
thermochronologic techniques have greatly increased our
isostatically
stablecrustof relativelynormalthickness.We
of the midcrustalprocesses
which play an integral
speculate
thatuplift andcrustalthinningpriorto HPG intrusion understanding
wastheresultof differentialthinningof thesubcrustal
lithosphere role in the growth and stabilizationof Precambriancrust [i.e.,
theHPGis spatiallyassociated
withthedeepest
exposed
Early
Proterozoic
country
rock.
Copyright
1997bytheAmerican
Geophysical
Union
Paper
number
97TC01629.
0278-7407/97/97TC-01629512.00
cooling has importantimplicationsfor crustalevolutionand
eratondevelopment
duringtheProterozoie[Bowringet al., 1996].
In constrast, very little is known about the timing or the
mechanismby which midcrustalEarly Proterozoicrocksof the
Black Hills were exhumedand cooled. Previousinvestigations
of
609
HOLMET AL.:SLOWMIDDLEPROTEROZOIC
COOLING,BLACKHILLS
610
i
i
i
• anorogenic
magmatism
(1.3-1.5
Ga)• posttectonic
magmatism
(1.7-1.8
Ga)
I
Q11.8-1.6
.'•-•l.0'(•a
collisional
belt
• Archcan
•l.'•
Ga
continental
rift
Ga
juvenile
crust
Provinces
O1.9-1.7Ga
collisional/accretionary
belts
ßI,
4,
4,
4,
4,
,I,
/
44 o
4,
,I,
4,
CENTRAL,'
PLAINS'"
/ / / /½,•I
/•,a./
/ / /
,, , ,, ,
/
/ / / / / /
, , , , , , ,
,,,,,,,,
ß ß ß
104 o
OUACHITAS
900
Figure1. ]:•'ecambdan
tectonicelements
of thesouthern
portionof theNorthAmericancraton(modJ•edafter
Jr'io.•mm'•
[1989b]). Insetshowsthelocadonwith• NorthAmerica. Abbreviations
•c asfollows:BH, BIack
Hills inlier andlocationof Figure2; GF, GreatFalls tectoniczone;and KR, Keweenawanrift.
large, well-datedgranite/pegmatite
intrusion,the 1710 Ma
HarneyPeak Granite(hereafterreferredto as HPG). The
widespread
thermaleffectsimposed
by thegraniteon thecountry
rocksthroughout
thesouthern
BlackHills arewell documented
frommetamorphic
studies
[Helmsandlz•tka, 1991;Terryand
In this study, we have used4øAr/39ArFriberg,1990;Fribergetal., 1996]. Becausethepre-1710Ma
theserocks have focusedprincipally on the tectonicsettingin
which they were initially formed at the surface and the
subsequent
conditionsand overalltectonicframeworkin which
theywere buriedandmetamorphosed
[e.g.,Reddenetal., 1990;
Terry and Friberg, 1990; Helms and Labotka,1991;Friberg et
at., 1996].
thermochronologyto investigate the postcollisionalcooling
historyof thisimportantEarly Proterozoicorogenicbelt.
Determiningthe initial coolinghistoryof Proterozoicorogenic
crustcan sometimesbe difficult becauseof thermaloverprinting
associatedwith youngereventsand/orrelativelypoorexposure.
Such has certainly been the case in the northern Black Hills
whereabundantTertiary igneousrockshave beenresponsible
for
at leastpartial resettingof Rb-Sr mineralages[Zartmanetal.,
1964]. In contrast,Tertiary igneousrocks are absentin the
southernBlack Hills, and Rb-Sr mineral ages are, for the most
part, substantially
older [Riley, 1970a;Walkeretal., 1986]. In
addition,rocks of the southernBlack Hills are relatively wellexposed and consist of coarse-grainedmetasedimentaryand
igneous rocks that contain abundantmuscovite and biotite.
Finally, the Early Proterozoicmetamorphismanddeformationin
the southernBlack Hills culminatedwith the eraplacementof a
metamorphic
history
of theregionisdifficulttoassess
because
of
thermaloverprinting,only the post-1710Ma, lower-temperature
coolinghistoryis addressed
here. We presentevidencefor
prol9nged
midcrustal
(3-4kbar)residence
andveryslowcooling
duringtheMiddleProterozoic.As for theEarlyProterozoic
accretiontry
beltsto the south,documentation
of a protracted
midcrustal
coolinghistoryhas,webelieve,important
implications
for both the Middle and Early Proterozoictectonothermal
evolution
of thisregionandfor genesis
of theposttectonic
Hamey
Peak Granite.
2. GeologicSetting
TheBlackHillscrystalline
coreconsists
of multiplydeformed
andmetamorphosed
EarlyProterozoic
rocksexposed
between
the
ArchcanSuperior
Provinceto theeastandtheArcbean
Wyoming
HOLM
ETAL.:
SLOW
MIDDLE
PROTEROZOIC
COOLING,
BLACK
HILLS
Province
to the west (Figure 1). Remnantsof Archeanrocksin
611
(FI) thatwererefolded
intoNNW strikinguprightfolds(F2)
theBlackHills (Figure2) suggestthis regionis partof the
accompanied
by thedevelopment
of a locallypenetrative
axial
planarfoliation
[Redden
andNorton,1975;DeWittetal., 1986].
Wyoming
cratonwhichhas beenreworkedduringEarly
Proterozoic
tectonism[Gosselinet al., 1988]. The basement Structures
developedduringD1 and D2 deformations
were
rocks
arecomposed
dominantly
of a thicksequence
of Early
modified
during
thesubsequent
mesozonal
granite
eraplacement
at 1710Ma [Reddenet al., 1990]. Structural
domingand
localizedfolding (F3) of metamorphiccountryrocks
tuffs
andgabbros
asyoung
as1883-1-5
Ma[Redden
etaL,1990], accompanied
granite
emplacement,
butthegranite
itselfislargely
wastectonically
buffed,deformed,
andmetamorphosed
priorto undeformed
[Redden
et aL, 1990]. However,
a late(post-HPG)
Proterozoicclastic metasedimentaryrocks and minor mafic
volcanic
andplutonicrocks. The entiresequence,
whichcontains
beingintrudedby theposttectonic
HPG at -1710 Ma [DeWittet
northeast
trending,
widespread
butnonpenetrative
foliation
(S4)
a/.,1986;TerryandFriberg,1990].Sillsfromthemaingranite ofuncertain
origin
hasbeen
recognized
byRedden
etal.[1990].
bodyhaveyieldedan Rb-Srwholerockisochron
of 1711-1-21
Ma
Of therocksexposed
throughout
theBlackHills,thesouthern
[Riley,1970a;Walkeret al., 1986] anda U-Pb monazitedateof
1715:k3
Ma [Redden
et aL, 1990]. Associated
pegmatites
which
surround
themainportionof theplutonyieldU-Pbapatitedates
of 1706+4.4
Ma to 1695-1-3
Ma [Krogstadand Walker,1994].
Thecrystalline
coreis nonconformably
overlainby Phanerozoic
regioncontains
thedeepest
crustal
levels.In thisregion,
widespread
metapelites,
amphibolites,
andquartzveinshave
yielded
metamorphic
pressures
ashighas5-7kbar[Terryand
Friberg,1990;Dukeet al., 1990a;Terryet al., 1994]. In
contrast,
metamorphic
pressures
to thenorthareinvariably
2-5
kbar[seeDukeet al., 1990b;KathandRedden,
1990].In the
south,the 1880-1710Ma, medium-pressure
regional
The main 1880-1710 Ma structuralelementsin the core of the
metamorphism
wasvariablyoverprinted
by a prograde
lowBlackHillsarenorthdirected,
ENE strikingfold nappes/thrustspressure,
high-temperature
contact
metamorphism
associated
with
sedimentary
rocks,indicatingits exposure
by Cambriantime,and
wasultimately
reexposed
byLaramidedoming.
ROCKER VILLE
15M []
42 B
BMD
LOCATION
PROTEROZOIC
77B
ROCKS
O RAPID
CITY
87 B,M
PEAK
AREA
•
GRANITE
Paleozoic
&Younger
Sedimentary
Rocks
Proterozoic Rocks
x x x x x x
S-107B,M
<vr• Hamey
Peak
Graniter-• &Metasedimentary
Rocks
MaficSchists(Undiff.)
:•:J;•
Muscovite&Biotite-Rich
Schist,
Undifferentiated
;..r'•Archean
Metasedimentary
Rocks
8,Granite
Sills
x;•Axial
Trace
ofSynform
LOCALITIES
OF
BIOTITE
(B)
AND MUSCOVITE (M)AGES
x"X,...
Lithologic
Contact ,&5- 13001500
Ma
1•"45" / / / / / / /
x'NFault
•1•61
•
1 18 >1800
Ma
•.
Isograd
- 1550 - 1650 Ma
Figure
2. Generalized
map
ofthesouthern
Black
Hills,
South
Dakota,
showing
locations
ofthemica-bearing
schists
andgranites
dated
inthisstudy
(geology
from
DeWitt
eta/.•[1989]
and
Redden
etal.[1990]).
Sample
location
symbols
arecoded
for4øAr/39Ar
mica
cooling
ages.
Muscoviteandbiotite-rich
schists
(units
Xmsand
XbsofDeWitt
etal.[1989])
delineate
thefault-bounded
synform
structure
situated
between
theHarney
Peak
dome
(HPD)
andtheBear
Mountain
dome
(BMD).GJFisGrand
Junction
Fault,
HCFisHillCityFault,
BFFis
Burnt
Fork
Fault,
andEMF.is
Empire
Mine
Fault.
Inset
shows
thelocation
ofthestudy
area
withrespect
tothe
crystalline
basement
exposure
of theentireBlackHills in southwestern
SouthDakota.
612
HOLM ET AL.: SLOW MIDDLE PROTEROZOIC COOLING, BLACK HILLS
emplacementof the HPG [Helmsand Labotka, 1991]. At the
Bear Mountaindome,for example,thermobarometry
indicates
peak metamorphic conditions for kyanite-bearing Early
Proterozoicmetasedimentary
rocksof 6.5 kbar andtemperatures
of-600øC [Terry and Friberg, 1990;Terry etal., 1994],with
garnetrimsyieldingfinal equilibration
pressures
of 4.4 kbarand
temperaturesof 530øC [Helms and Labotka, 1991]. Also,
metapelitesfrom the HarneyPeakdomeyield peakmetamorphic
conditions of 4.5-5.0 kbar and ~620øC and garnet rim
equilibration
conditions
of 3.0-3.5 kbarand-550øC [Terry,!990;
Helms and Labotka, 1991].
Isotopic data indicatethat the interior granitesof the HPG
plutonwerederivedfromvapor-absent
meltingof biotitein deepseatedArchean/Proterozoic
metasedimentary
rocks[Walkeret al.,
1986; Nabelek et al., 1992a, b; Nabelek, 1994].
Abundant
mineralogicand thermobarometricdata from the countryrock
exposedaroundthe pluton indicatefinal emplacementat 3-4
kbars [Redden etal., 1985; Helms and Labotka, 1991]. Both
geophysicaldata and geologicmappingindicatethat the HPG
consists
of a series
of sheet-like
intrusions
with
a base at
relatively shallow (less than a few kilometers) crustaldepths
[Reddeneta/., 1982; Duke etaI., 1990a; Klasner and King,
1990]. Althoughnot exposedin the Bear Mountaindomearea,
geophysicaldata suggestHPG exists there not far below the
surface[Duke et aI., 1990a; see also DeWitt eta/., 1989]. Field
mappingsuggeststhat the rocks betweenthe Harney Peak and
Bear Mountain domeswere structurallyinvertedfrom a D2
antiforminto a synformwhen the HPG was emplaced[DeWitt et
a/., 1989].
4. Methodology
In order to determine the cooling age pattern acrossthe
southern Black Hills, samples of the HPG and the
metasedimentary
rocks were selectedfrom widely spaced
localities(Figure2). Eighteenmedium-grained
peliticschists
and
metagraywackes
were sampledfrom the garnet,staurolite,and
sillimanitezonessurrounding
the HPG [Dahl etal., I993].
Petrographic
analysisrevealsthat the micasare typical!y
unaltered, with only minor hematite or chlorite alterationof
biotite. The majority of the rocks sampled contain one welldevelopedfoliation. In the majority of samples,biotiteis
somewhatcoatsetgrainedthancoexistingmuscovite(i.e., 0.1-0.4
x 0.0!-0.02 mm for muscovite versus0.1-0.5 x 0.01-0.05 nun for
biotite). In someof the samples(S-107, S-103, ST-107, andST-
112),late, coarser-grained
(0.3-0.5x 0.1-0.2 mm),randomly
orientedmicasovergrowthe primaryfoliation. Theselate micas
constitute<5% (by volume) of total muscoviteor biotite in each
sample.
Mica separates were obtained using standard magnetic
separationtechniquescombined with paper separationand
handpicking.The coarsestpossiblegrain sizelackingcomposite
grains(usuallythe 250-180I.tm range)was chosenfor dating.
Purityof approximately99% wasobtainedby theseprocedures,
as verified by petrographieexamination and confirmedby
InductivelyCoupledPlasma(ICP) analysis[Dahl eta/., 1993].
Samplepreparation,irradiation,and analyticalprocedures
for
nøAr/•OAr
incremental
releasedatingfollowstheprocedures
describedby Lux [1986]. Sampleswere encapsulated
in tin foil
and irradiated in the L67 facility of the Ford Reactor at the
Phoenix Memorial Laboratory reactor of the University of
Michigan. Variations in neutron flux during irradiationwere
3. PreviousThermochronology
monitoredwith the Universityof Maine flux monitorSBG-7 (age
Wetherillet aI. [1956]obtainedK-At ageson HarneyPeak = 240.9 Ma relative to MMhb-1 (519.5 Ma) [Alexanderet aI.,
1978]). The sampleswere heatedin a molybdenumcrucible
pegmatitemineralsfrom the Bob IngersollMine locatednorthof
using
radio frequency induction within an ultrahigh vacuum
theHameyPeakdome(Figure1). Usingthedecayconstants
of
system on a line to a Nuclide model 6-60-SGA mass
Steigerand Jdger[1977], theirdatagavedatesof 1510Ma for
muscovite,1360 Ma for lepidolite,and 1080 Ma for microcline. spectrometer.Sampleswere analyzedby the incrementalheating
technique in which the sample is heated repeatedly at
Gotdichet al. [1966] reportedtwo (recalculated)
K-At datesfor
successivelyhigher temperatures. Results are presentedas
muscovite from the HPG of 1570 Ma and 1610 Ma.
Riley [1970a, b] recognizeddiscrepancies
in Rb-Sr mineral release spectra in which the horizontal width of each box
datesfromthe HPG andits associated
pegmatites.Pegmatite representsthe size of an incrementrelative to the others,and the
heightrepresentsthe uncertaintyassociatedwith eachapparent
micas, microcline, and poilucite gave dates from 1575 Ma to
1695Mawhenaninitial87Sr/86Sr
Of0.7!4wasassumed
[Walker age. Plateau ages (designatedas Tp) were calculatedfrom
consecutivegasincrementsthat togetherconstitute>50% of the
et al., 1986]. A similarRb-Sr dateof !680-2:25
Ma (2{I) on
total gas released. A 95% confidencelimit using only the
muscovitefrom boudinfracturefillings near the flank of Bear
analytical uncertaintywas the basis for determiningwhether
Mountaindome [Rattd, 1986] is commonlyinterpretedto
consecutive
incrementsoverlappedin age. Age uncertainties
representthe approximatetime of domeformation[Reddenetal.,
were calculatedby the methoddescribedby Dalrympleetal.
1990; Terry and Friberg, 1990]. However,Rattdand Zartman
[1981], arereportedat the 2t3level, andincludethe uncertainty
in
[1970] alsoreporteda muchyoungermuscoviteRb-Srdateof
the flux measurement(J value). Analytical resultsfor each
1560Ma from a muscoviteschistsamplecollectednortheast
of
the Bear Mountain dome.
sample
aregivenin theAppendix
•.
Morerecently,Reddenetal. [1990]obtainedRb-Srwholerock 5. Results
isochrondatesfrom a metatuffanda metagraywacke
collected
Fourteen
muscovite
and!2 biotiteseparates
weredatedin this
fromthecentralandnorthernBlackHills. The metatuffgavea
study,
including
24
mica
separates
from
the
metasedimentary
1534:!:50
Ma (20) dateandthe metagraywacke
gavea 1572.-k89
Ma (2•) date. Reddeneta/. [ 1990]tentatively
suggested
thatthe •Supporting
dataareavailable
withentirearticle
onmicrofiche.
pooledage of 1543:1:43
Ma (2•) for thesetwo samples
may Orderby mailfromAGU, 2000FloridaAve.,NW, Washington,
represent
the timeof formationof theweak,northeast
trending, DC 20009 or by phoneat 800-966-2481' $2.50. Document
S4 foliationfoundthroughout
muchof theBlackHills.
97TC01629M.Payment
mustaccompany
order.
HOLM ET AL.: SLOWMIDDLEPROTEROZOIC
COOLING,BLACKHILLS
rocks
whichsurround
theHPGand2 muscovite
separates
from
thegranite
itself.Spectra
wereobtained
from6 micapairsof
schist
andmetagraywacke.
Thereleasespectra
of all 26 mica
separates
areshown
in Figure
3. In Figure
3, andin thetext
following,
thesamplenumber
for themetasedimentary
unitsis
preceded
byletters
which
represent
themetamorphic
grade
ofthe
613
ST-112)yieldedconcordant
micapairdatessuggesting
coolingof
theserocksthroughthe averagemica closureintervalbetween
1620 and 1640 Ma. Three othersamples(S-107, ST-87, andST47) yieldedbiotite datessomewhatolder thanthoseof coexisting
muscovite. We surmise that the reverse age discordance
exhibitedby thesemica pairs probably reflectsincorporationof
smallamounts
of unresolved
excess
4øAt.Preferential
sample
(GZ,garnet
zone;
ST,staurolite
zone;
S,sillimanite
zone; relatively
aøArintobiotiteovercoexisting
muscovite
is
andKY, kyanitezone). Plateauand near-plateau
datesare intakeof excess
interpreted
to record cooling throughclosureinterval well known [Brewer, 1969] and appearsto be related to the
temperatures
required
for4øArretention.
It haslongbeen
existence of weaker K-O bonds in biotite relative to muscovite
recognized
thatcoolingratemarkedlyaffectstheintegrated
[DAM, 19961.
closure
temperature
[Dodson,1973]. Althoughthe nominal
closure
temperature
for Ar diffusionin muscovite
is oftencitedas
ca.350øC[blcDougalland Harrison, 1988], studiesin slowly
5.2. Middle
cooled
terraneshavecalculatedintegratedclosuretemperatures
as
Proterozolc
dates
Six muscovite and 5 biotite separatesobtained from 10
samplesof the metasedimentaryrocks collectedin the domal
lowas-295øCandrim closuretemperatures
of -205øC [Hodges
regions
yielded
MiddleProterozoic
4øAr/39Ar
dates
between
1270
etaL, 1994]. In light of theevidencefor slowcoolingpresented and -1560 Ma (Figure2). Two dateswere also obtainedfrom
below,
we assume
integrated
mica(muscovite
andbiotite)closure
samplesof theHPG collectednearHarneyPeak. Muscovitefrom
a coarse-grainedsample (HPG-1) yielded a plateau date of
Although
lessthanhalf of thetotalsamples
datedyieldstrict 1403:i:8 Ma. Approximately 3 km to the east-southeast,
plateaus
(Figure3), we notethatmostof thesamples
arewell muscovitebooksfrom a pegmatiticphaseof the graniteyieldeda
behaved
in thattheyyieldyoungageincrements
onlyfor thefirst somewhatdiscordantage spectrum with a total-gas date of
5-10%of the gas releasedand then level off to form near 1456:i:.34
Ma. The differencein the grainsizesof thesesamples
plateaus.
Also,thereisthesameamount
of agevariability
across might be a factorthatcontributedto the differencein their dates
theregionregardless
of whetherall the dataor only theplateau andstyleof releasepatterns.
datesare considered. Becauseof thesepoints,in the following
temperatures
in therangeof 250ø-300øC.
discussion
we interpretessentiallyall of thedata(bothtotalgas
andplateaudates)as informativeand representative
of the
regional
coolinghistory.
Datesrangefromasold as-1818 Ma to asyoungas-1270 Ma
with considerablescatter in between. However, most dates fall
6. Discussion
We have obtained 26 dates for minerals from 20 different rock
samplescollectedover much of the southernBlack Hills. There
is a generalpatternof relatively old, Early Proterozoic(>1600
Ma) datesexistingbetweenthe two domalregionsand southof
Bear Mountain dome (Figure 2), althoughthere is considerable
scatterin the age data (Figure 4). We have acknowledgedthe
intooneof two agerangecategories(Figure4). For instance,12
country
rockmicaseparates
yieldrelativelyold dates(1600-1655
Ma), whereassevenseparates(granite and countryrock) yield
dates
between1440 Ma and 1510 Ma. For a givenmineral,there
likelypresence
ofsome
excess
4øAr
intheolder
population
butdo
isnorelationbetweenageandphysicalgrainsize. For example, not considerit probablethat the entire older populationis the
biotites
fromsamplesST~42andST-87 haveessentially
thesame result
of swamping
byexcess
nøArandtherefore
meaningless.
physical
grainsize but havea nearly200 Ma age discordance. The spectraare, for the most part, well behaved from both
Also,biotitefrom sampleST-88 is coarserthanbiotitefrom
muscovite and biotite separates, and the dates obtained are
87(located
only2 km away)butyieldsa datewhichis about50
geologically
reasonable.
We interpretthe 1600-1655Ma datesas
Mayounger.One biotiteseparateobtainedfrom schistcollected representingthe time of initial cooling of theserocks through
justnortheastof the HPG (ST-18) yielded an apparentage of
averageAr mica retentionconditions. Thesedatesare over 501818:k12
Ma. Abundantgeothermometric
dataindicatethatthese 100 m.y. youngerthanthe -1710 Ma HPG and thereforearetoo
rocks
wereheatedto temperatures
above550øCwhenthe-1710
youngto reflectsimplecoolingfollowinga heatingeventat 1710
Ma HPG intruded[Helms and Labotka, 1991; Friberg et al.,
Ma [Carslaw and Jaeger, 1959]. If the countryrock into which
1996]. This fact, togetherwith the fact that surrounding the graniteintrudedwas cold (i.e., <300øC),then the thermal
muscovite
datesare all 1400~1500Ma (samplesHPG-1, HPG-5,
perturbationof the geothermcreatedby the intrusionwould
GZ-11, and GZ-15), indicates that the 1818 Ma date is
dissipatequicklyandthe resetmica agesshouldbe close(within
unreallstically
oldandprobably
affected
byexcess
4øAt.Finally, 10-20 m.y.) to the ageof theintrusion.
fora givenmineralthereis norelationbetween
ageandpresence
Instead, ambient country rock temperaturesat the time of
or absenceof late porphyroblasts,in part becausethese graniteintrusionmusthave been aboveconditionssufficientfor
porphyroblasts
constitute
only a smallmodalpercentage
of the
muscovite
orbiotitepopulations
dated.
$.1.EarlyProterozolc
(1600-1655Ma) dates
Sixmuscovite
and6 biotiteseparates
fromthesouthern
Black
Hillsyielded
EarlyProterozoic
aøAr/39Ar
dates
between
1600and
1655Ma (Figure3). Mostof thesedateswereobtained
from
samples
locatedbetween
thetwodomalregionsandalsosouthof
the
Bear
Mountain
dome
(Figure
2)..Twosamples
(ST-107
and
diffusive
lossof nøAr
overgeologic
timescales
(i.e.,above-250
ø350øC),andwe associate
the coolingof theserocksat 1655-1600
Ma to postintrusionregional uplift. Becauseof the similar
averageretentiontemperatures
for muscoviteandbiotite,we are
unableto ascertainwith certaintywhetherthis periodof uplift
was rapid or slow. Similar plateau and total gas datesfrom
coexistingmuscoviteandbiotite (i.e., in samplesST-107 andST112) using the incrementalheating method do not necessarily
imply rapidcooling[seeHodgeseta/., 1994]. In fact, the -50
614
HOLM ET AL.: SLOW MIDDLE PROTEROZOICCOOLING,BLACK HILLS
I!
II
I
I-.I-
c::)
HOLM ET AL.: SLOW MIDDLE PROTEROZOICCOOLING,BLACK HILLS
Proterozoicplutons (Figure 1).
615
Admittedly, there are no
basementexposuresfor a considerabledistancesouth of the
Zo
I--
1850
I
'1 I
1650
I
!•1
lelelO
Ioleld ele
I I
1450
1250
AGE (Ma)
muscoviteAr/Ar age
biotiteAr/Ar age
Black Hills, andit is possiblethat Middle Proterozoic
plutons
might extendclose to the Black Hills given their apparent
preferencefor intrudinginto Proterozoiccrust (Figure 1).
However,if ourMiddleProterozoic
datesfromthedomalregions
(andsimilardatesfromthe northernBlackHills [Gardneret al.,
1996]) were interpretedto representa widespreadthermal
resetting
event,it wouldbe difficultto explainwhyrockslocated
betweenthe domal regions and also southof Bear Mountain
domewereselectively
not reset. In addition,EarlyProterozoic
crustinvadedby Middle Proterozoicplutonsin the southwestern
Figure
4. Summary
histogram
ofmean
4øAr/39Ar
mica
agedata UnitedStates
yieldshornblende
4øAr?gAr
dates
of 1430-1350
Ma
fromthesouthernBlack Hills. Early Proterozoicdatesareon the
leftandMiddle Proterozoicdatesare on thefight.
m.y.rangein micacoolingagessuggests
thatit wasprobably
not
rapid;rapidcoolingof a regionshouldresultin uniformages
from different localities [Hobn and Lux, 1996]. In addition,
preliminaryinvestigationssuggestthat within this older
population
mica age may vary with composition(e.g., with
gg/(Fe+Mg)in biotiteandK/(Na+K) in muscovite).If real,this
wouldfurtherindicatethat coolingand uplift were slow during
indicatingMiddle Proterozoicregional metamorphism[i.e.,
Karlstromet al., 1997]. The lack of any Middle Proterozoic
hornblende
datesfromtheBlackHills [Berryet al., 1994;Dahl et
a/., 1996]mightthereforebe considered
consistent
with the lack
of evidence
for MiddleProterozoic
plutonism
in thearea.
We believetheyoungerMiddleProterozoic
micadatesmaybe
bestinterpreted
asevidenceof slowisobariccoolingrelatedto
relaxationof an elevatedgeothermor asevidenceof a periodof
slow Middle Proterozoic(1500-1300Ma) uplift. The regional
patternof 1400 to 1600 Ma Rb-Sr and K-Ar biotite agesfrom
rocksin the southernthird of the WyomingProvincehavelong
thistimeperiodas composition-controlled
age variationswould
beeninterpretedto reflect Middle Proterozoicuplift [Peterman
onlybedetectedin slowlycooledterranes[Dahl, 1996].
and Hildreth, 1978; Karlstrom and Houston, 1984].
As noted earlier, petrographic analysis of some of the
Thermochronologic
data from provincesfarther southwestin
metasedimentary
rocks we dated (samplesST-107, ST-112, and
Arizonaindicatethat terraneassemblyat 1700 Ma wasfollowed
S-107)revealstwo generations
of muscovite,one in theprimary
by a >200 m.y. stableperiodof little to no uplift [Karlstromand
(S2) foliation and one occurring as coarser, late, randomly
Bowring,1993]. This stableperiodwasthenfollowedby slow,
oriented
porphyroblasts.
The latemuscovitewidelyrecognized
in
regionaluplift beginningat about1450 Ma andcontinuingfor at
thesouthern
Black Hills is commonlyinterpretedto represent
least several hundredmillion years [Bowring and Karlstrom,
growthof that mineralduringthe low-P, high-T metamorphism
1990; Karlstromand Bowring, 1993]. Indeed, the timing of
associated
with intrusionof the HPG. However, Berry et al.
intrusionand coolingproposedhere for the HPG is remarkably
[1994]haveproposedrecentlythat at leastsomenewgrowthof
comparableto the cooling history recently proposedfor the
muscovite
occurredduringa younger,low-temperature
thermal
-1700 Ma Crazy Basin pluton of centralArizona. Substantial
eventbelow the biotite closure temperature. In this study,
Middle Proterozoicagegradientshavebeenobtainedfrom Crazy
muscovite
and biotite datesfrom rock samplescontaininglate
Basin pluton muscovitecrystals using both laser spot-fusion
muscovite
porphyroblasts
are analyticallyidentical,and thereis
mapping(--400 m.y. core-m-rimage variations[Hodgeset at.,
noconstraint
to suggest
thatlatemuscovitegrewbelowthebiotite
1994]) andfurnacestepheating[Heizler and Raiser, 1996]. The
closure
temperature.Ourresultsthereforeareconsistent
with the
100-200m.y. age gradientscommonlyexhibitedby our Middle
common
interpretation
of late muscovitegrowthbeingrelatedto
Proterozoic
mica spectra(Figure3) andthe -200 m.y. spreadin
intrusionof the HPG.
Middle Proterozoicdatesare togetherstrongevidencefor slow
Mica4øAr/39Ar
datesfromthedomalregions,
fromboth
unreformed
samplesof the HPG and fromthe metasedimentary
cooling.
countryrock, are consistentlyMiddle Proterozoic. They are
similar
tomuscovite
4øAr/39Ar
datesobtained
byBerryet al.
7. Interpretations of Map View Age Patterns
[1994]fromcountryrockcollectednortheast
of the granite.
7.1. Retention Variation
Except
forsample
ST-47,ourdatagiveMiddleProterozoic
dates
forbothbiotiteandmuscovite.
An important
question
iswhether
We havenotedabovethe possibilitythatthe agescatterwithin
theseyoungerdatesare the resultof a superposed
thermal the older population might reflect composition-controlled
resetting
eventor whethertheyreflectthetimeof initialcooling variations in Ar retention intervals, and we wonder if the overall
of theserocksduringupliftor slowisobaricrelaxation
of the mapviewagepatternseenin Figure2 mightalsoreflectvariation
geotherm.
in retention intervals on a larger scale.
Actual closure
It is well knownthatvoluminous
MiddleProterozoic
(1500-
temperature
intervalsof mineralsare influencedby factorssuch
1300Ma),midcrustal
plutons
wereemplaced
alonga northeast as effectivediffusiondimension(physicalgrain size?),mineral
trending
transcontinental
beltextending
fromsouthern
California composition,
andcoolingrate. The apparentlack of correlation
toLabrador
[Anderson,
1983]. We suggest,
however,
thatour betweenage and physical grain size noted aboveleadsus to
Middle
Proterozoic
micaagesareprobably
nottheresultof a speculatethat subgraindomains(boundedby dislocations,
pluton-related
thermal
resetting
eventconsidering
thattheBlack cleavages,alteration phases,etc.) variably governedeffective
Hillsarelocated
over200km northof anyidentified
Middle diffusiondimension(and thusrelative coolingage) amongour
616
HOLM ET AL.: SLOWMIDDLE PROTEROZOICCOOLING,BLACK HILLS
Retention variation model
W crustal
surface E
i
a)
~350oC
BMD
.....
b)
W
:
d)
~300oc
crustal
surface E
BMD
-1700
Ma
',,.%' HPG
~350øC
'•....
•HPG
C) W
W
700 Ma
crustal
surface E
~300øC
Foldingof isothermsmodel
e)
-1 600 Ma
~300oc•
....
..•
-1400
Ma
' '•:" ';::,'HPG
•.Current
level
of exposure
crustal
surface E
~300øC
f)
crustal
surface
E
W
w
~1600
Ma
crustal
surface
E
oC•_
_. •
---1
400 Ma
",S• HPG
•',•.Current
level
of exposure
Figure 5. Simple schematiccross-sectional
(upper crust only) synopsesdepictingtwo end-member
interpretations
for themicaagepatternpreserved
in thesouthern
BlackHills. Bothmodelsbeginat -1700 Ma
shortlyafter the intrusionof the HarneyPeakGranite(HPG) which causeddomingof the surroundingrock
(creatinga synformbetweenthe domes,S). In the retentionvariationmodel (left), long-termmidcrustal
residenceand very slowcooling(associated
with eitherdownwardmotionof horizontalisothermsor very slow
subhorizontal
uplift of theregion)occursfor severalhundreds
of millionyearsafterintrusionof theHPG. The
differentcoolingagesresultfrom differences
in Ar retentionintervalsbetweenthe synform(S) region(shaded)
and domalregions,not differencesin structuraldepth. (a) At-1700 Ma, all micasare abovetheir retention
intervalsin the midcrust.(b) By -1600 Ma, slowcoolingis recordedin themorehighlyretentivemica minerals
of the synformregion. (c) At -1400 Ma, micasfrom thedomalregionscoolbelowtheirretentionintervals. In
thefoldingof isothermsmodel(right),thedifferentlow-temperature
(350ø-300øC)historiesfor rockswithinand
without the synform(S) are interpretedto be the resultof broadMiddle Proterozoicfolding after a period of
protractedthermalequi!ibration
in themiddlecrustfollowingHPG intrusion.Currentlyexposedrockswithin the
synformare denotedwith a solid circle. (d) Intrusionof the -1710 Ma Harney Peak Granite into deformed
countryrock with ambienttemperatures
somewhatabove350øC. (e) Minor uplift during the next 100 m.y.
resultsin minor slow coolingof the rockscurrentlyexposedin the synform. (f) Approximatelyeast-west
orientedcompression
beginningat -1500 Ma resultsin reactivation
andaccentuation
of the synformandfolding
of the 1500 Ma -300øC isotherm.This foldingresultsin upliftandcoolingof therocksoutsideof the synform
andjuxtapositionof relativelycoldrockswithinthe synform.BMD is BearMountaindome.
micas. A retention-composition
relationshipis suggested
within
our dataset by the fact thatrelativelyold biotitesandmuscovites
from the synform(Figure 2) are alsoenrichedin Mg/(Fe + Mg)
anddepletedin K/(Na + K), respectively,relativeto micasin the
domalregions[Dahl et al., 1993]. Sucha correlationappears
broadly consistentwith crystal-chemical predictions [Dahl,
uplift/coolinghistory. Full treatmentof this matteris currently
underwaybut beyondthe scopeof this paper. For now, we
acknowledge
that the synformregion exhibitsdifferences
in
lithology,mica composition,and possiblyeffectivediffusion
dimension
in micasdistinctfrom the surrounding
domalregions.
Giventhesedifferences,
the agepatternmightthenbe theresult
1996].
of slow subhorizontaluplift and cooling of mineralswith
Isolating the potential contributionsof microstructureand
differentAx retentionintervals(i.e., higherretentionintervals
compositionon Ar retentionin our micas is an exceedingly withinthe synformregionand southof BearMountaindome).
difficulttaskwhichcanonlybe approached
through
outcrop-scale Thisexplanation
for theEarly andMiddle Proterozoic
micaage
dating studiesdesignedto eliminatethe regionalvariable of
patternis depicted
on theleft sideof Figure5 (retention
variatio,
HOLM ET AL.: SLOWMIDDLEPROTEROZOIC
COOLING,BLACKIIK,LS
617
model).Immediately
afterintrusionof the HPG, all rocks ultimatelyverified,then the region affectedby the compression
currently
exposed
in the southern
BlackHills areat ambient may be substantiallylarger than originally proposed[see also
temperatures
slightlyabove-350øC(the nominalclosure
temperature
of muscovite,
Figure5a). By z,1600Ma,.the
crust
Fueten and Redmond,1997].
hascooled
uniformally
(asshownby thedeepening
of horizontal 8. Implicationsfor the Early Proterozoic
isotherms).
Synform
rockswiththeirhigherAr retention
interval
(say
~350
øC)would
haveclosed
todiffusion
ofArbythistime, 8.1 Early ProterozolcGeothermalGradient
whereasdomal rocks with lower Ax retention intervals (say
~300øC)remainedopen to Ar diffusion(Figure 5b). Slow
cooling
continues
for about200 m.y. until at -1400 Ma the
currentlevel of exposureof the domal regions cools below
~300øC
(Figure5c).
7.2. Foldingof Isotherms
The mica '•øAr/39Ar
datesobtainedhereindicatethat the HPG
did not cool throughthe muscoviteAr retentionintervaluntil
200-300 m.y. after intrusion. Thus ambient country rock
temperatures
at thedepthandtime of intrusionof theHPG were
warmer than ~300ø-350øC. Abundant geothermometriedata
indicatethatthe countryrock surrounding
the granitewasheated
by thegraniteto temperatures
above550øC[Friberget al., 1996].
However,
countryrock
hornblende
'*øAr/39Ar
agesareconcordant
if we assumethat micas in the southernBlack Hills all have
(withinerror)with theplutonage[Berryeta/., 1994;Dahl and
essentially
similarretentionintervals,then the map view age
Holm, 1996; DaM et al., 1996]. This indicates that after the
heatingevent,the countryrockscooledquicklyto
pattern
mightbe interpreted
to be theresultof broadMiddle pluton-related
Proterozoicfolding after a period of protracted thermal
temperatures
below 500øC (the closuretemperaturewidely
assumedfor hornblende),suggestingthat the ambienttemperature
of 3-4 kbar countryrocksprior to intrusionof the granitewas
below500øC. Garnetrim temperatures
of 530ø-550øCassociated
with HPG intrusion[Helms and Labotka, 1991; Friberg et al.,
1996] in the countryrock surroundingthe granitehave been
by some[e.g.,Nabeleketa/., 1992a]asindicatingthe
current
preservation
of oldermicadatesbetweenthedomescould interpreted
ambienttemperatureat the level of graniteeraplacement
(3-4
reflectreactivationof a preexisting Early Proterozoicfaultare
bounded
synformalstructure(and down warpingof relatively kbar). However, we emphasizethat theserim temperatures
associated
with
heating
of
the
country
rock
by
the
pluton
(which
coldrocks)duringa periodof approximately
east-west-oriented
the ambientgeothermalgradient)andthereforecannot
contraction
andslowuplift duringtheMiddleProterozoic
(Figure perturbed
50. To thesouthof thedomeswheredatesareolderin thewest be usedas an estimateof the ambientgeothermalgradientat (or
andwherethe synformstructureis absent,the sameisotherm just priorto) thetimeof HPG emplacement.
Takentogether,the hornblendeand mica age datafrom the
mightbeonlybroadlywarpedanddip gentlyto thewest. The
factthat the map patternof young-to-old-to-young
mica ages southernBlack Hills indicate that the country rock temperature
coincides
with a preexisting
mappedEarly Proterozoicsynform was between 350øC and 500øC at the 12-14 k.m depth of
eraplacement
of the HPG, suggesting
an average,uppercrustal,
[DeWittet al., 1989] couldbe taken as strongevidencethat the
geothermal
gradientof 25ø-40øC/km
justpriorto intrusion.This
structure
wasreactivatedduringthe MiddleProterozoic.
We emphasizethat this interpretationfor the agepatterndoes determination is somewhat lower than the 40ø-45øC/km
geothermal
gradientestimated
by Nabe!eketa/. [ 1992a]on the
not rexlUkemajor crustal deformationor large amountsof
differentialdisplacementalong the synformboundingfaults basisof oxygenisotopedata. We concludethat the average
duringtheMiddle Proterozoic(Figure5f). We alsonotethatthe
geothermalgradientin the southernBlack Hills was not
highjustpriorto graniteintrusionandthatthelower
synform
boundingfaults[DeWittet al., 1989]betweenthedomal abnormally
regionscrosscutonly Early Proterozoicmetamorphicrocks. crustal anatexis (which resulted in the HPG magma) was the
Otherthanthis simplecrosscutting
relation,thereare no field result of a temporarytectonic perturbationof an average
constraints
indicating that these faults could not have been geotherm.
equilibration
in themiddlecrustfollowing
HPGintrusion.
In this
scenario,
we interpretthe oldermica datespreservedbetweenthe
domalregionsand southof Bear Mountain as representing
shallowercrustalregions which had already cooledprior to
renewedMiddle Proterozoicuplift (Figures5d and 5e). The
reactivatedat some later time. Indeed, without information such
as the thermochronologicdata presentedhere, it is nearly
impossible
to ascertain
thetimingof motionon thesefaultsfrom
field data alone.
8.2. Early ProterozoleP-T Evolution
As describedabove,peakcountryrockmetamorphic
pressures
Historically,the Middle Proterozoicera in North Americahas
beeninterpretedas a periodof profuseanorogenic
plutonism
associated
with extension[WindIcy,1993]. We haveraisedthe
enticing
interpretation
thatthe coolingagepatternin thesouthern
of 5-6.5 kbar in the southern Black Hills contrast with final
Christensen,
1995;Kirbyeta/., 1995;Gonzales
et al., 1996].If
intrusion.In addition,we disagreewith this interpretationfor two
equilibrationpressures
of 3-4 kbar [Terry and Friberg, 1990;
Helms and Labotka, 1991; Terry et al., 1994]. The peak
metamorphicpressureshave been interpretedby some as
theinitialconditions
of eraplacement
of theadjacent
BlackHills might reflect Middle Proterozoicmidcrustal representing
shortening.Althoughit remainsto be shownwhetherthis HPG. For instance, Terry et al. [1994] envision HPG
eraplacement
(beginning
at depths>22 kin) asconcomitant
with
interpretation
(or the alternativeretentionvariationmodel)is
correct,it is at least consistent with several recent studies from
significant structural doming and regional uplift to final
eraplacement
depthsof 12-14k.m. We notethatto ourknowledge
thesouthwestern
UnitedStateswhichprovideevidence
for a
existwhichrequirelarge amounts(>8-10
widespread
midcrustal
contractile
deformational
eventduringthe no timingconstraints
MiddleProterozoic
[Nymanet al., 1994;Duebendorfer
and km or more) of uplift anddomingto haveoccurredtogetherwith
618
HOLM ET AL.: SLOW MIDDLE PROTEROZOIC COOLING, BLACK HILLS
reasons. First, in section 8.1 we noted that the 3-4 kbar country
rocks now exposedat the surfacewere probablycoolerthan
~500øCprior to graniteintrusion;ambienttemperatures
thislow
wouldnot havebeenlikely at -22 kin. Second,theexistence
of
prograde, low-P, metamorphic isograds (staurolite+biotite,
andalusite+biotite,
and sillimanite)surrounding
theHPG [Helms
andLabotka,1991] alsosuggestthatthecountryrockmusthave
been relatively cold (-350ø-500øC) when the granite was
emplaced.A simpleuplift/coolingP-T pathfrompeak(>22 km)
to garnet-rim(12-14km) pressures
solelyduringgraniteintrusion
would not allow for the developmentof suchlow-P, prograde
Labotka,
1991].Whileit iswelldocumented
thatsome
doming
anduplift.occurred
witheraplacement
of theHPG[Redden
etal.,
1985],for theabovereasons
we considerit unlikelyto havebeen
of theorderof 8-10 km or more. We proposeinsteadthatmostof
theupliftof thecountry
rockfrom20-25kmdepths
(ormore)
occurred
priorto eraplacement
of theHPG at 12-14km depths.
In thisscenario(Figure6), the HPG intrudedrelativelycoldrocks
in the southernBlack Hills, imposed the prograde thermal
isogradssurrounding
it (Figure2), andcooledquicklyto ambient
countryrock temperatures
(Figure6). The HPG stabilized
and
resided in the midcrust where it cooled slowly for several
hundreds
of millionyears. We notethatthe P-T pathsproposed
here are very similar in characterto the looping P-T paths
proposedfor Proterozoicmidcrustalrocks of the southwestern
metamorphicisograds.
8.3. ProposedProterozoic P-T Paths
UnitedStates[WilliamsandKarlstrom,1996].
In Figure6, we reconstruct
two separate
P-T pathsfor rocksof
the southernBlack Hills: one for low-P metamorphicrocks
surrounding
the HPG eastof the synformandonefor medium-P
kyanite bearingrocks of the Bear Mountain dome west of the
synform (Figure 2). Tectonic burial of Early Proterozoicrift
sedimentsas youngas -1880 Ma indicatescrustalthickeningand
heatingpost-1880Ma. Resultsof recentgarnetandstaurolitePbPb step-leachdatingof Bear Mountaindomerockssuggests
the
collision and attendantpeak metamorphismoccurredat about
1760 Ma [Dahl and Frei, 1997]. At that time, palcopressures
were at or above 6.5 kbar in the Bear Mountain dome and 4.$-5
kbar in the Harney Peak dome (Figure6). Abundantbarometric
data from garnet rims throughout the southernBlack Hills
indicate emplacement of the HPG at 3-4 kbar [Helms and
8.4. Speculations
on the Origin of the Harney PeakMagma
The thermochronologic
datapresentedhere suggestthatafter
Early Proterozoiccollisionand intrusionof the HPG the areaof
the southernBlack Hills did not undergorapid uplift. Instead,
intrusion was followed by a prolonged period of tectonic
quiescenceand midcrustal residencefrom -1700 to 1500 Ma
during which probablyno or perhapsonly minor slow uplift
occurred. We propose thereforethat the HPG magmawas
emplaeed into isostatically stable crust of relatively normal
thickness. Since rifting occurredprior to collision in the Black
Hills, the subsequent
collisionof thinnedcrustmay haveresulted
in onlymoderatelythickcrust[cf. BowringandKarlstrom,1990].
BMD path
Ma peak of
regional metamorphism
KY
6
SIL
plift prior to
HPG intrusion
HPD path
1710 Ma plutonenhanced metamorphism
st a
2
880 Ma crustal
thickenir and heating
300
500
T, øC
SIL
700
Figure6. Proposed
pressure-temperature-time
paths
formedium-pressure
andlow-pressure
EarlyProterozoic
metasedimentary
rocks
inthesouthern
BlackHills,South
Dakota.Aluminosilicate
triplepointisafterHoldaway
[1971].Abbreviations
st,a, ands represent
staurolite+biotite,
andalusite+biotite,
andsillimanite
isograds,
respectively.
BMD is BearMountain
dome;HPDis HarneyPeakdome.Solidsquares
represent
peak
metamorphic
conditions
(afterTerryet al. [1994]for BMD andTerry[1990]for HPD) andgarnet-rim
thermobarometric
data(afterHelms
andLabotka,
[1991]).Solidcircles
represent
a -200 m.y.periodoftectonic
quiescence
andstability
afteremplacement
ofthe,--1710
MaHarney
PeakGranite.
Slowcooling
through
-300øC
occurred
at 1500-1300
Maforbothpaths.Seetextforfurther
explanation.
HOLM ET AL.: SLOWMIDDLE PROTEROZOIC
COOLING,BLACKHILLS
619
Table1. Interpretation
ofthe1880-1400
Ma Geologic
Historyof theBlackHills, SouthDakota
Event
Absolute
age,
Reference
Ma
Rifting
Thrustingandnappe
folding(S 1)
Isoclinalfolding(S2)
andmetamorphism
Subcrustal
lithospheric
thinning/crustaluplift
Doming and intrusion
of the HarneyPeak
-1880
Reddeneta/. [ 1990]
<1800
Dahl [1993]
1740-1760
just prior to
1710(?)
~1710
Dahl eta/. [1996]
Dah! and Frei [1997]
This study
Reddenetal. [1990]
Granite with formation
of S3 andlate
muscoviteovergrowths
Tectonicquiescence
with
minoruplift; weak $4
foliationmay have
formedduring this time
Slowmidcrustalcooling
possiblyassociatedwith
1710-1500
~1400
This study
Reddenetal. [ 1990]
This study
contraction
Giventhis,the8-10 km of uplift whichwe proposeaboveto have
preceded
HPG intrusionmay have been enoughto returnthe
moderately
thick crustto a relatively normalthickness
prior to
emplacement
of the HPG.
Thegeneration
of the HPG magmais commonlyattributed
to
partialmeltingof the lower crustin responseto extremecrustal
thickeningand thermal relaxation [Helms and Labotka, !991;
Nabelek
et al., 1992a,hi. However,thelackof substantial
rapid
postintrusion
upliftsuggests
isostatically
stablecrustof relatively
normalthickness[of. Karlstromand Bowring, 1993]. As an
alternative
possibility,
Simsetal. [1991]haveproposed
thatthe
and both may have been initiated by a deeper-seated
process
perhapsassociatedwith subcrustallithosphericthinning(i.e.,
mantle delamination). One consequenceof delaminationis
isostaticuplift and thinning of the overlying crust. A second
consequence
is an increasein mantle heat flux acrossthe Moho
(by shallowingof the asthenosphere-lithosphere
boundary)and
lowercrustalmelting[Kay and Kay, 1993]. Given an appropriate
time lag between these two responses,delamination might
providean explanationfor why coolingof uppercrustalrocksis
followedby fusionof the lower crustandmelt eraplacement
into
stable midcrustal rock.
Postcollisional
mantle delamination
has
HPGmightberelatedto thesuturing
of therocksof theCentral been proposedon the basisof geophysicalevidencefor portions
Plains
OrogenalongtheCheyenne
Belt. Whilethishypothesis of the Trans-Hudsonorogento the north(centralSaskatchewan,
eliminates
theneedfor partialmeltingof anoverthickened
crust, Canada)wherepostcollisionalmagmasexist [Baird etak, 1995]
it is inconsistent
with boththe timingandthe well-documented and has beensuggestedas a possiblecausefor genesisof postsouthward
dip of the Cheyennesuture[Duebendorfer and collisionalgranitesin the Early ProterozoiePenokeanorogenof
Houston,
1987].
the southernLake Superiorregion (Figure 1) [Holm and Lux,
As notedin the geologicsettingabove,in the wholeof the 1996]. Whateverthe mechanismfor posttectonicgranitegenesis,
BlackHillsthe HPG is spatiallyassociated
with thedeepest whetherby delaminationor thermalrelaxation[Windley, 1993],
exposed
EarlyProterozoic
countryrock. Micacoolingagesin the
northern
BlackHills [Gardneretal., 1996]arecomparable
to the
southern
BlackHills, andthereforethespatialassociation
does
notappearto be relatedto greateruplift of the southern
region
during
or sincethe Middle Proterozoic.We haveestablished
in
as with youngercollisionalbelts,we envisioncrustalthickening
followedby crustalthinningwith generationanderaplacement
of
late-collisionalor anatecticgranite to be intimatelyrelated[e.g.,
Burchfieletal., 1992; MoInar and Lyon-Caen,1988; Turner et
a/.,•1992].
thispaper
thatEarlyProterozoic
postcollisional
upliftandcooling
ofthecountry
rockoccurred
priorto midcrustal
3-4kbargranite
eraplacement,
andtherefore
thisspatialassociation
is notrelated
9. Summary and Conclusions
toin situbiotitedehydration
meltingof the deepest
exposed
portion
of theorogen.Although
thetimingof EarlyProterozoic Themica4øAr?øAr
agedatah'omthesouthern
BlackHills
unroofing
from --6 kbar pressuresto 3-4 kbar pressures
is
uncertain,
we speculatethat this uplift, and the generation
and
eraplacement
of granite
whichfollowedit, aregenetically
linked,
depicta simplemap view patternin whichrelativelyold dates
(1600-1655 Ma) are preservedwithin the synformstructure
locatedbetweenthe HarneyPeak andBearMountaindomesand
620
It(3LM ET AL.: SLOW MIDDLE PROTEROZOIC COOLING, BLACK HILLS
also southof Bear PvI0mntain. In contrast, Middle Proterozoic
thinned
during
aperiod
ofuplift,4) Last,wespeculate
thatEarly
upliftandcrustalthinningandsubsequent
generation
mica dates(--1500-1 270 Ma) areobtainedfrom the surrounding Proterozoic
domalregions. The dat• are interpretedto indicatea periodof
oftheHPGmagma
mayhavebeen
theresult
oflocalized
thinning
subcrustal
lithospherebeneaththe southern
low-•ernperature•
co<•lia
.gassociated
with eitherslowMiddle of the overthickened
Proterozoi½
isobaricc0•liag or uplift at -1300-1500 Ma. The
map view age patternrmay reflectessentiallyhorizontalslow
coolingof an area•rlaichtextaibits
variationsin micaAr retention
intervals.Thepatterra
rainyaltemativelybe explainedasreflecting
a mildly folded lV'Iidc•l½
Proterozoic (-1500 Ma) ~300øC
isotherm. If so, sh•rtel•ing may have been localizedalong a
majorpreexistingnoxth-•0uthorientedsynformalstructure
andis
consistent
w'itha rcggi0n•lMiddle Proterozoicwest-to-northwest
contractilee•ventrecentlyproposedfor westernNorth America
[Nyrnanetal., 1994]_
In additionto pr0•idingex,idencefor slowMiddleProterozoic
cooling,thexnicaagedataprovideimportantimplications
for the
Early Proterozoicte•ct0raothermal
historyof the southern
Black
Hills (Table1). 'These
iraclucte
thefollowing:(1) Intrusionof the
HPG wasfollowedby a prolonged
period(-200 m.y.) of crustal
stabilityand teet0nicqmies•ence,
2) The HPG magmaintruded
into crustof relativelyla0rrmalthickness,3) Prior to intrusion,
crust overthieken½cld•ring collision (after-1880 Ma) was
Black Hills.
Finally, we notethat at -1710 Ma, the crustin the southern
BlackHills wasprobablynot60-70 km thickasmightbeinferred
if the 12-14 km of crust that originally overlay the HPG were
addedto the current45-55 km crustalthickness.Instead,crustal
structure
hasprobably
beenthickened
sincetheEarlyPmterozoic,
eitherby magmaticunderplatingduring the Middle Proterozoic
togetherwith thickening
dueto shortening
and/orpossibly
even
by muchyoungerLaramidetectonismandmagmatism.
Acknowledgments.We thankE.T. GardnerandD.A. Schneider
for
insightfuldiscussion
andassistance
in the datacollection.We thankB.
Bauer,E. Duke,L. Friberg,P. Nabelek,J. Redden,andM. Terryfor
numerousdiscussions
regardingthe geologyof the Black Hills; E.
DeWitt, J. Redden,and M. Heizler for comments;and K. Chamberlain
andW. Hames
forconstructive
reviews.Thisworkwassupported
inpart
by anNSFgrant(EAR93-040780)
andby two KentStateUniversity
ResearchCouncilgrants.
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(ReceivedAugust21, 1996;
revisedMay 1, 1997;
acceptedMay 14, 1997.)